and thank you in advance for the hand-holding.

I was troubleshooting a very small and noisy signal in a circuit and the concern of probe loading came up...

I only have access to affordable passive probes like the these ones available from Tektronix so buying a $1K active probe designed specially for measuring small signals isn't feasible. I was instructed to create a simple unity gain amplifier circuit and connect the input to the part of the circuit being measured, and the output to the scope probe to attempt to get a better oscilloscope measurement.

It's sounds reasonable, unity gain buffers should be capable of isolating the impedance of the probe and isolate the effect of probe loading on the circuit I am trying to measure. But I just haven't seen it anywhere on the internet before, as in an engineering blog or troubleshooting app note that says something like: "here's what happens when a simple LM358 is used to improve your small noisy signals on your oscilloscope readings".

Can someone please provide some examples, and that the results are reliable? Tangent question, If it's so easy why don't passive probes already have this as an option?

Thanks again.

  • 1
    \$\begingroup\$ You mention small signal and loading. Is it one or the other or both? What noise level and maximum acceptable loading (resistance + shunt capacitance) are you targeting? It would be a very, very crummy scope that could be improved with an $0.01 op amp like an LM358, though if you're interested in low bandwidth small signals a limited bandwidth op ampl might help (eg. up to 100Hz 100uV). \$\endgroup\$ – Spehro Pefhany Oct 14 '15 at 0:17
  • \$\begingroup\$ Do you have an estimate of the bandwidth and amplitude of the signal you are trying to capture, and how much loading it can tolerate? Also, you can avoid loading the signal by putting a resistor in series with your oscilloscope probe. Of course this will also low pass filter the signal, but depending on what you are doing, maybe you don't care. \$\endgroup\$ – mkeith Oct 14 '15 at 2:22

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